Interleukin-2 (IL-2) was identified almost 30 years ago [1] and originally called T cell growth factor because of its ability to stimulate T lymphocytes in vitro. It is a protein with a reported molecular weight of between approx. 13 kda and 17 kda [2] and an isoelectric point of approx. 6 to 8.7.
IL-2 has been used in the clinic for boosting effector immune responses in cancers and infectious diseases [3, 4]. It is now authorized for use in human for the treatment of cancer.
In one of its registered indications, adjunct treatment of renal cell carcinoma (RCC), less than 10% of the patients respond to treatment. This limited efficacy of IL-2 is now partly explained by the recent discovery that IL-2 also plays a major role in the peripheral survival and suppressive function of regulatory T cells (Tregs) [5, 6], which are known to supress anti-tumor effector responses.
In fact, IL-2/IL-2 receptor (IL-2R) signalling is important during immune responses of both effector T cells (Teff) and Treg. On the one hand, extensive IL-2R signalling is necessary for the development of terminally differential short-lived Teff cells that exhibit enhanced functional activity, and for eliciting proper T cell memory [7]. On the other hand, IL-2/IL-2R signalling is essential for Treg development and homeostasis as shown by the fact IL-2 knock-out mice lack Tregs. Noteworthy, IL-2 or IL-2R deficient mice are able to mount effector immune responses, as notably attested by their development of severe T-cell mediated auto-immune diseases (AID).
These different consequences of IL-2 signalling abnormalities are now explained by the fact that both quantitative and qualitative differences in IL-2/IL-2R signalling regulate Treg and Teff. Tregs appear to require low IL-2/IL-2R signalling threshold to support their development and peripheral homeostasis [6]. IL-2 administration has been shown to lead to marked expansion and activation of Tregs in mice and humans [3, 4, 8].
Nowadays, IL-2 continues to be utilized exclusively for cancer immunotherapy, and has not been investigated in human auto-immune diseases or, more generally, in human diseases caused by an undesirable immune response. This is because of the perceived and expected risks associated with such treatment. Indeed, the capacity of IL-2 to stimulate Teffs carries the risk of activating the very effector T cells that mediate the disease and therefore to aggravate the disease.